Ceramic filter media are commonly used in a wide range of fluid handling procedures, including filtration, diffusion, recovery, transfer, mixing and foaming. Ceramic filter media are also employed as catalysts themselves, or as carriers for catalysts. Ceramics are well known to possess several advantages as filter media over alternatives such as organic or metallic filter media. One such advantage is that ceramics generally possess superior resistance to deterioration from heat or chemical exposure, in comparison to other media.
Ceramic filter media are most often used in the form of an aggregation of ceramic particles, either loose or bound to one another. The ceramic particles can be formed as spheres, platelets or needles. The particles are routinely obtained by crushing and classifying (that is, sorting by size) a previously manufactured mass of the desired material. This method of manufacture is subject to some drawbacks, however. Crushing of the formed materials can often degrade some of the desirable structural characteristics enjoyed by the material, such as its impact strength, mechanical strength, rigidity, porosity or aspect ratio. With respect to materials such as platelets or needles, a lower aspect ratio indicates either a shorter or a thicker needle. Thinner needles yield filters with smaller pore sizes. Moreover, with materials such as platelets or needles, the separately formed platelets or needles are often very delicate and interwoven when manufactured, and consequently fracture upon attempts to separate and classify them. Even if they can be successfully separated, aggregation of the particles may require sintering or the use of a bonding agent for rigidity, impact strength or mechanical strength. Alternatively, the particles can be aggregated by placing them in a metal container. However, such containers can be expensive, and may not be completely resistant to the gas or liquid being treated.
Ceramic filter media have also been used as supports for discriminating layers such as fluorocarbon polymers or sintered ceramic membranes. These supports have typically been made from previously fired spherical particles of alpha-alumina or cordierite. The particles are then lightly sintered to bond them together and give them mechanical strength. Unfortunately, the resulting supports may not possess all the strength that might be desired, particularly against impact, or against the pressure of the fluid flowing through them. Moreover, the resulting supports have not been very porous, usually only about 30 percent porous (or 70 percent of theoretical density, defined as 100 percent minus the volume percent porosity).
Many solutions have been suggested to these problems. Each entails its own drawbacks, however. For example, published Japanese Patent application JP 63-103877A (Nagasaki Ken, published May 9, 1988) discloses a process for preparing a porous compact useful for industrial filtering, for foam generating, as a bioreactor carrier, or for catalysis. The compact is described as having fine porous structure with a relatively high deflection strength. The compact consists of acicular mullite crystals formed from compression molding and sintering stoichiometric mullite (3 Al.sub.2 O.sub.3.2SiO.sub.2). The starting material includes additives so as to allow transfer of any unreacted or any excess silica into a glass phase, which is then eluted with an acid.
U.S. Pat. No. 3,993,449 (Howard Jacobson et al., Nov. 23, 1976) discloses a process for preparing single crystal mullite fibrils useful as fillers, catalysts, or catalyst supports. The fibrils are made from aluminum sulfate, a silica source and an alkali metal salt (fluxing agent). The molar ratio of aluminum to silicon in the reactants is from 2.6:2 to 6:2, expressed as Al.sub.2 O.sub.3 /SiO.sub.2, with at least one alkali metal atom for each aluminum atom. The reference states that although a product that is predominantly "true" (3/2 or stoichiometric) mullite can be obtained from reactants throughout that range, it is preferred to maintain the ratio in the range 2.8 to 3.4:2, so as to avoid the quantity of alpha-alumina platelets which are obtained if there is a large excess of alumina in the reactant mixture.
U.S. Pat. No. 3,881,944 (George H. Beall et al., May 6, 1975) discloses a porous glass-ceramic body, for example, of siliceous glass and mullite, from which the glass is leached by hydrofluoric acid. The predominant phase of the body can be a sub-stoichiometric mullite, which may be no more than 25 percent crystalline, with excess SiO.sub.2 and a modifying oxide. The highest degree of porosity obtained in the examples after leaching is 56.5 percent.
While each of these three disclosures suggests that a fibrous mullite body or support can be obtained that is relatively strong, it is not clear that the whiskers of the bodies are bound to each other sufficiently to provide the impact and mechanical strength necessary for use of the bodies in a variety of environments. Moreover, control of pore size in such bodies is not as great as could be desired because the average pore sizes are typically quite small. The bodies are thus often not useful for applications requiring higher porosities, for example, from 50 up to 85 percent. The use of hydrogen fluoride to elute the glass phase is itself inconvenient because of the risks involved in handling hydrofluoric acid. Additionally, devices constructed from metal generally cannot be used in processes employing hydrofluoric acid as an elution agent.
It is therefore an object of the present invention to provide a ceramic filter structure with a discriminating layer thereon, where the filtering medium has been grown in situ to form a network of interlocked needles or platelets which has high mechanical strength, high impact strength, heat resistance and good resistance to thermal cycling.
It is a further object of the present invention to provide a support for a ceramic or other filter membrane having high porosity, yet with superior bonding of the material making up the support for the filter.
It is yet another object of the present invention to provide a method for manufacturing such a filter including a ceramic support which does not entail the dangers associated with the prior use of hydrogen fluoride or other acid for eluting a glass phase from the support.
It is a further object of the present invention to provide a method of interlocking two or more pieces into a ceramic support having a uniform composition and structure, including throughout the locations at which the pieces are joined.
It is also an object of the present invention to eliminate the need for elements of disparate composition, and the need for any means to connect such elements, in a ceramic filter structure.